I passed through a lot of states on my trip to and from California. On the way there, taking mostly interstates 70 and 15:
Then on the way back, taking mostly interstates 40 and 81:
Texas (just the panhandle)
West Virginia (just a corner!)
Maryland (even smaller corner, I think a truck blocked my view)
...and then I was back into Pennsylvania and New York, where I'd started out. Twenty states in all!
In other travels previously, I'd been to Vermont, New Hampshire, Maine (as a kid? not certain), Massachusetts, Georgia and Florida. So all together, I've been in or through 26 states. That's half.
While I'm no fan of sitting in the car all day, I really like watching landscapes and climates change as I drive along. I'm kind of interested now in doing a more northern and a more southern route if I drive it again next fall, although since I liked the highway 40 route so much I'd be most likely to just do that again, as anything else would be adding a lot of distance to the trip. Route 40 isn't the shortest route, but it had a great mix of landscapes, was far enough south to avoid bad weather in February for the most part, and didn't cross too many big cities, no huge ones. The highway 70 route was OK too, but it crossed a lot of big mid-western cities, and a lot of the landscape in the first half of the drive looked an awful lot like Canada.